Most simplistically it has been known to protect ferrous surfaces by mixing hexavalent chromium compounds and silicate materials in the same coating composition. These can typically be emulsions containing resinous materials. Emulsives may include polyacrylic acid, and coating operations can proceed in conventional manner to achieve corrosion protection for the ferrous surface.
A more complex approach to the protection of substrates has attempted to involve the combination of particulate aluminum plus zinc metals, along with a silicate in a single coating composition. Occasionally the use of some form of chromium is employed, but this can require the deletion of one or more of the first mentioned ingredients.
Specifically, in providing for the more complex systems, it has been found suitable to add to aqueous solutions of alkali metal silicate containing dissolved chromate, the ingredients aluminum powder and zinc oxide. That is, particulate zinc is not employed. Rather, zinc oxide is found compatible. Coated metals, particularly ferrous metals, after coating and heat curing of applied coating, are found to have corrosion and abrasion resistance, as taught in U.S. Pat. No. 3,656,975.
If the deletion of chromium is considered, then it has been taught in U.S. Pat. No. 3,423,229 to combine pulverulent zinc and aluminum in a coating composition of soluble metal silicate. After application, the coating can be cured to water insolubility by the subsequent application of an acid solution. Or, if careful attention is directed to the proper mix of aluminum and zinc powder, and these are further combined with a C.sub.1-4 -alkyl silicate, in appropriate proportion, the coating can be heat cured. On topcoating with a finishing powder paint, a coating of heat resistance plus water resistance is taught in U.S. Pat. No. 4,229,495.
Looking more particularly at two coat systems, it has been known in the protection of zinc surfaces to first treat the surface with a traditional chromate undercoating, and then topcoat the treated surface with colloidal silicas or silicate solutions. This protection against white rusting can be obtained with films such as from silicate solutions of sodium silicate and/or potassium. In addition to retarding white rusting, the topcoating can also retard staining as has been discussed in Japanese Patent Disclosure No.: Showa 53-125239.
In focusing on undercoatings, it is known to use substantially resin free, chromium-containing coatings for protecting ferrous substrates. Of special interest are those which contain particulate metal. Representative coating compositions can have a portion of the chromic acid constituent replaced by a boric acid component as disclosed in U.S. Pat. No. 4,266,975. Other complex compositions such as shown in U.S. Pat. No. 3,907,608 and which may contain the pulverulent metal and hexavalent-chromium-providing substance in a liquid medium comprising water plus high-boiling organic liquid, have been recently shown to be most useful coatings over ferrous surfaces when provided with a silicate topcoating, as has been disclosed in U.S. Pat. No. 4,365,003.